QmlDesigner: Check if view is attached

Task-number: QDS-4382
Change-Id: Ic44d1fad30c5cec65be2cce611fbb37b62c69a6f
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

src/plugins/qmldesigner/components/propertyeditor/itemfiltermodel.cpp

@@ -92,7 +92,7 @@ QVariant ItemFilterModel::modelNodeBackend() const
 
 void ItemFilterModel::setupModel()
 {
-    if (!m_modelNode.isValid())
+    if (!m_modelNode.isValid() || !m_modelNode.view()->isAttached())
         return;
 
     m_lock = true;